The use of chemical sensing devices to detect certain gases is known. Many attempts have been made to find a material with selectivity and sensitivity for a specific gas. For example, U.S. Pat. No. 4,535,316 discloses a resistive sensor for measuring oxygen. See also H. Meixner et al, Sensors and Actuators, B 33 (1996) 198-202. It is apparent that different materials must be used for each gas to be detected. However, when a gas is part of a multi-component system, using one material to detect a specific gas is difficult because of the cross-sensitivities of the material to the various component gases of the mixture.
One example of a multi-component gaseous system is a combustion gas emission, which can include oxygen, carbon monoxide, nitrogen oxides, hydrocarbons, CO2, H2S, sulfur dioxide, hydrogen, water vapor, halogens and ammonia. See H. Meixner et al, Fresenius' J. Anal. Chem., 348 (1994) 536-541. In many combustion processes, there is a need to determine whether the gas emissions meet requirements established by federal and state air quality regulations in various jurisdictions. Several types of gas sensors have been developed to address this need. See U.S. Pat. No. 5,630,920, Friese et al, which discloses an electrochemical oxygen sensor; U.S. Pat. No. 4,770,760, Noda et al, which discloses a sensor for detecting oxygen and oxides of nitrogen; and U.S. Pat. No. 4,535,316, which discloses a resistive sensor for measuring oxygen. It would be advantageous to be able to simultaneously analyze two or more components of a mixture such as a combustion gas emission, to calculate concentration for example, in terms only of data generated by direct contact of the gases with a sensor and without having to separate any of the gases in the mixture. Prior art methods do not currently meet this need.
Numerous sensors have been disclosed to detect gases evolving from foods and from other relatively low temperature applications. See K. Albert et al, Chem. Rev., 200 (2000) 2595-2626. Arrays of several undoped and doped tin oxide sensors have also been disclosed for use in detecting various combustion gases up to 450° C. See C. Di Natale et al, Sensors and Actuators, B 20 (1994) 217-224; J. Getino et al, Sensors and Actuators, B33 (1996) 128-133; and C. Di Natale et al, Sensors and Actuators, B 23 (1995) 187-191. However, at higher temperatures and in the highly corrosive environment in which one would use chemical sensors to monitor combustion gases, operating temperature can alter or impair the performance of the sensor array. That being the case, high temperature environments require the use of materials that are both chemically and thermally stable and that maintain measurable responses to the gases of interest. The effect of the operating temperature on the response of tin oxide bases sensor arrays was studied up to 450° C. See C. Di Natale, Sensors and Actuators B23 (1995) 187-191. However, materials in addition to those previously known in the art are still needed to be able to provide a method and apparatus capable of directly monitoring the gas emissions of multi-component gas systems at higher temperatures, such as would be encountered in the operation of combustion gas systems.
Addressing this need would permit the use of a chemical sensor to measure combustion emissions, such as automobile exhausts, and determine whether those emissions meet functional and mandated requirements. In addition, it has surprisingly been found that the method and apparatus of this invention that are useful for analyzing high temperature gases, such as automotive emissions, may be employed with equal effect in analyzing low temperature gases.